1. Field of Invention
This invention relates to an improved method of manufacture of color filters suitable for such uses as flat panel displays.
2. Description of Related Art
Liquid Crystal Displays (LCD) have been used for many years in place of cathode ray tubes (CRT) screens for small and large sized displays. However, LCD usage has been limited to high cost applications (such as expensive laptop computers) due to the high cost of fabrication. Recent improvements have permitted development of large size, high resolution displays which are useful in notebook and desktop computers. Such LCD panels, particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future. Such display panels may take two forms: passive matrix and active matrix liquid crystal displays (AMLCDs). Passive matrix displays employ transparent electrodes patterned in perpendicular striped arrays on facing glass plates, that is superimposed one on the other. Red, green and blue color filters on the inner surface of one of the glass plates provide the full color display. The passive matrix display is ostensibly easier to fabricate than AMLCDs, but is much more limited in performance capabilities.
One of the challenges to reducing the cost of fabricating thin film transistor (TFT), also known as an active matrix, displays is the color filter, which can cost up to 25% of the total LCD cost. In these devices, white light passes through a light valve (the TFT LCD) which adjusts the intensity of the light and then the intensity adjusted light passes through a color filter to give the desired color. A pixel is made up of three colors (each with an independent light valve) corresponding to the primary colors. Accordingly, white color results when the filters are full on; and black color results when all of the filters are full off. The resolution and alignment of the color filter must be such that the filter overlays the TFT devices exactly and provide a very clean differentiation between colors of the mask. Furthermore, the color of the elements in the filter must be consistent from one filter to another and be within a narrow color tolerance. Other types of displays, such as plasma filters, can use the color filter embodied in the present invention.
The fabrication of an active matrix liquid crystal display involves several steps. The assembly comprises two glass panels, identified as front and rear panels. In the first step, the front glass panel is prepared, which involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (red, green and blue) color patterns within the spaces outlined by the black matrix. The color elements are each typically about 70 to 100 microns in width by 200 to 300 microns in length. These dimensions are typically used for notebook computer applications. The front glass substrate is completed by deposition of a transparent conducting layer over the color filter element.
Although the present invention also is suitable for use in passive liquid crystal displays, it will be described in embodiments of an active display and specifically a thin film transistor (TFT) liquid crystal display. As partially shown in FIG. 1, a conventional TFT display 10 comprises an array of cells or pixels A, each cell including a thin film transistor 11 to address the cell by applying a voltage to the cell when the transistor is in its on state and a capacitor 12 which maintains the voltage after the transistor is switched off. The transistor is formed on a glass substrate 13 on the back side of the display 10 and is connected between a column or data electrode 14 and a row electrode 15 and to a display transparent electrode 16 of each pixel, all at the back side of the display 10. The front side of the display 10 is formed with a continuous common transparent electrode 17 which is spaced apart from and positioned parallel to the transparent display electrode. Both the common electrode 17 and the display electrode 16 are preferably formed of a thin transparent conductive material, such as indium tin oxide (ITO), carried on a glass substrate. Since the display electrode of each pixel is smaller in dimensions than the continuous common electrode, a fringe field results which spreads outward from the pixel or cell edges of the display electrode to the common electrode when voltage is applied across the electrodes. Parallel with the outside of the common electrode 17 and adjacent glass substrate 18 is a polarizer 19, which is appropriately orientated relative the a polarizer 20 mounted in back of the rear glass substrate 13. Alignment layers 21 and 22 are disposed on the inner surface of the display and common electrodes 16 and 17, respectively, and are in contact with a liquid crystal layer 23, herein twisted nematic liquid crystal molecules with a positive dielectric anisotropy, which is sealed between the two parallel mounted glass substrates carrying the alignment layers 21 and 22. On the back side of the display 10 is a visible light source (not shown) which irradiates the display 10 through a diffuser 24. If it is desired to have the display 10 in color, a color filter 25 is disposed adjacent the non-alignment layer side of the common electrode 17, and contains groups of the three primary colors (red, green, and blue), each one of the primary colors being associated with one of a group of three adjacent pixels A to form a color cell.
To illustrate the environment of the present invention in more detail, FIG. 2 shows an enlarged cross-section of the layers of a single domain cell or pixel (prior art) of the liquid crystal display taken along line 1--1 of FIG. 1. with switch 26 (representing the TFT in each pixel) open and a voltage is not applied across the liquid crystal layer 23. In this illustration, the liquid crystal layer comprises twisted nematic liquid crystals with a left-handed twist which is conventionally achieved by using chiral additives. FIG. 2 diagrammatically shows this LC layer 23 as elongated molecules 28a, 28b, 28c, 28d, 28e, 28f, 28g, 28h, 28i, 28j, 28k, and 28l with molecules 28a, 28b, 28c, and 28d being in contact with surface 29 of the front alignment layer 22 and molecules 28i, 28j, 28k, and 28l being in contact with surface 30 of the rear or back alignment layer 21. Molecules 28a-d and molecules 28i-l are tilted longitudinally away from their respective surfaces 29, 30 by the same angle a0. Because of the twist angle of the LC molecules, the molecules along the surfaces 29 and 30 are drawn going into and out of the plane of the paper. The bulk molecules, as depicted by 28e-28h, are drawn longer since they are oriented more parallel to the plane of the paper. Surface 29 of the front alignment layer 22 is disposed adjacent the transparent electrode 17, the color filter 25, which is optional, the glass substrate 18, and the polarizer 19 in that order. Surface 30 of the rear alignment layer 21 is disposed adjacent the transparent electrode 16, and the glass substrate 13, the polarizer 20, and the diffuser 24 in that order. The light on the back side of the diffuser 24 for irradiating the liquid crystal display panel is not shown. When switch 26 is closed as shown by the dashed line 26a and voltage is applied, the molecules 28a-d and 28i-l on alignment surfaces 29 and 30 which are influenced by the same pre-tilt angle a0 cause the bulk molecules, as shown by the center molecules 28e-h, to move in the direction as shown by the dashed arrows 31.
In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel.
The third and final step is the assembly of the front and rear panels, including injection of a liquid crystal material between the two panels to form the liquid crystal cell.
Ideally, in LCD displays, the transparent conducting layer, which typically is indium tin oxide (ITO), should be as smooth as possible to ensure electrical continuity. In addition, any thickness variations in the glass substrates or coatings can result in visible defects in the final display. Consequently, it is also important that the liquid crystal layer that fills the gap between the front and back panels be as uniform as possible across the entire display.
Because the glass substrate which forms the front panel is itself a relatively flat article having parallel sides, any variations in thickness usually occur as a result of the process used to deposit the color filter array. It is therefore desirable to deposit color filter patterns which have a smooth upper surface and as uniform a thickness as is possible, because once a uniform thickness color filter/substrate composite has been obtained, it is a relatively straight forward process to deposit a smooth ITO layer and obtain a uniform cell gap when the front panel is combined with the rear panel.
For this reason, photolithographic techniques are now preferred over printing techniques for forming color filters, because photolithography is capable of forming uniform color arrays. Nonetheless, all the deposition methods used thus far, including photolithography, by themselves have not been capable of depositing sufficiently smooth color patterns. Consequently, a planarizing layer is commonly applied over the color patterns to alleviate any imperfections in coating smoothness or thickness uniformity due to the deposition process. The transparent planarizing layer also serves to protect against ion migration to or from the ITO layer and color pattern layer. The planarizing layer should also be as smooth and flat as possible.
To facilitate deposition of the flat planarizing layer noted above, it is desirable that the color patterns be as smooth, flat and substantially parallel to the undersurface of the glass substrate. Also, color patterns of uniform cross-section are desirable for obtaining optimum display contrast and color performance, because if the thickness of the pattern varies, the transmitted light intensity will vary.
One method heretofore used to form color filters is photolithography, in which each color pattern in the color filter is deposited in a separate step. As mentioned above, photolithography has, in the past, been a preferred method of depositing color filters, especially when compared to ink printing methods such as waterless lithography, gravure and typography. Photolithography is preferred because it deposits image dots having a more flattened, rectangular cross-section.
The printed ink dot, on the other hand, typically has a more round-topped or triangular cross-section due to surface tension effects. In addition, in typical printing processes, because the ink tends to wet both surfaces during a transfer from roll to roll or from roll to substrate, the inks tend to split cohesively to some extent during such transfers. This may further contribute to non-uniformity of the ink dot thickness, particularly for high viscosity inks. This results in an ink dot which, when deposited onto a substrate and cured, has a non-uniform cross-sectional shape, and this in turn results in an uneven surface which is more difficult to alleviate using a planarizing layer.
In addition, photolithographic printing methods are inherently more accurately registered because the alignment between different color patterns is accomplished by optical rather than mechanical methods, and optical methods are intrinsically more precise. For all of these reasons, various prior workers in the flat panel display art have concluded that printing methods are substantially inferior for making color filters for LCD panels.
For example, the authors of Color Filter for Liquid Crystal Display by Ueyama et al, SEMI-SEMICON/West 92, International Flat Panel Display Conference, Section B, Pages 41-59, explain that, while printing methods are less expensive, the accuracy of ink printing methods is not sufficiently reliable to make high quality color filter components. The article points out, as also mentioned hereinabove, that printing methods are thought to be quite inferior in quality compared to photolithography, primarily because of the rounded cross-sectional shape of printed dots.
K. Mizuno and S. Okazaki, in The Japanese Journal Of Applied Physics, Vol. 30, No. 118, November, 1991, pp. 3313-3317, proposed producing a color filter by a process wherein ink patterns are successively prepared on a transfer (offset) roll and cured by exposure to ultraviolet light (UV) prior to transfer to the substrate. Each cured ink color pattern is individually transferred to a glass substrate coated with an adhesive layer.
U.S. Pat. No. 4,445,432 discloses a method and apparatus, relevant to a different art, for applying thermoplastic decorative inks onto various substrates by printing each color ink onto a releasing surface from a heated engraved or etched metal surface, transferring the various colors from each releasing surface onto a second releasing collector surface to form a multi-colored print, and transferring the multi-colored print to a ceramic, glass-ceramic or glass substrate. Various color inks are successively printed onto a collector roll, after which the resultant pattern is transferred to the substrate. Such processes have not been used to make color filter patterns.
U.S. Pat. No. 4,549,928 (Blanding et al.) describes using a similar technique for printing phosphors and a black matrix onto color TV panels. In this operation, thermoplastic pressure-sensitive inks, corresponding to the red, green and blue color phosphors and the black matrix, are applied separately to the collector roll to form the desired pattern. This pattern is then transferred to the TV tube panel.
Unfortunately, all of the techniques described above result in the ink dots having the conventional rounded or triangular cross section. It would be desirable to develop a method which results in smoother, more uniform ink dot shapes which are more suitable for color filter array applications.
In addition, color filter arrays typically undergo rather severe potentially destructive heating and treatment steps during manufacture of the LCD display. For example, the transparent conducting layer, typically indium tin oxide (ITO), is usually vacuum sputtered over the color filter array panel. This commonly takes place at temperatures elevated as high as 250_C., for times which may be as long as one hour or more. Also, the liquid crystal is assembled by laminating the front and rear glass panels under pressure with thermally curable adhesives, which typically require temperatures in excess of 200.sub.- C. Not all materials can withstand such high temperatures.